Raw water intake blowing bubbles?

[prof_mariner]

This one's got me scratching my head. I have a Perkins 4108 fresh water cooled diesel in a 41 foot sailboat. I've not had this boat long so only noticed this recently, but really don't know how long it's been doing this and I've not seen this on other boats. When I'm running the engine, I noticed that the raw water intake (at the very bottom of the boat) seems to be blowing bubbles into the ocean. The intake is not shared with anything else, it goes directly to the engine mounted, belt driven pump and then into the engine exhaust. Plenty of water comes out the exhaust, the intake strainer is clear, and the engine runs cool (doesn't overheat). I changed the RW pump's impeller not too long ago so I doubt that's the cause, but that's the last thing I can think of. Other ideas?

[S/V Antares]

Are you seeing bubbles in the Sea water Strainer that could be caused by loose hose or strainer fittings?

or bubbles comming out the thru hull on the bottom of the boat? Is this with the engine running? (Very unlikely)

[prof_mariner]

Originally Posted by S/V Antares

Are you seeing bubbles in the Sea water Strainer that could be caused by loose hose or strainer fittings?

or bubbles comming out the thru hull on the bottom of the boat? Is this with the engine running? (Very unlikely)

It's coming out the thru hull on the bottom of the boat. Weird, huh? Like I said, it's a real head scratcher. I would think that if the pump wasn't working properly it would just pump less water. I guess I'll have to pull the RW pump apart again to see what's up. Hate doing it if I don't have to as it's in a real awkward place to get at. But I can't think of anything else.

[model 10]

You sure the bubbles are not going in, not out?

[Stu Jackson]

Originally Posted by prof_mariner

It's coming out the thru hull on the bottom of the boat. Weird, huh? Like I said, it's a real head scratcher. I would think that if the pump wasn't working properly it would just pump less water. I guess I'll have to pull the RW pump apart again to see what's up. Hate doing it if I don't have to as it's in a real awkward place to get at. But I can't think of anything else.

Why? If the rw pump is working, and you say it is, why not look at the condition of the thru hull and its attachments? It sure sounds like more is going on at the thru hull than the pump. You could, for instance, take the hose off the strainer inside the boat and run it into a bucket (keep filling the bucket with a hose from the dock) and make sure the rw pump is working properly. That would eliminate the pump. Seems like some kind of leak at the thru hull - loose fitting itself, bad strainer connection to the seacock, etc.

Good luck, happy forensics.

[a64pilot]

Originally Posted by Stu Jackson

Why? If the rw pump is working, and you say it is, why not look at the condition of the thru hull and its attachments? It sure sounds like more is going on at the thru hull than the pump. You could, for instance, take the hose off the strainer inside the boat and run it into a bucket (keep filling the bucket with a hose from the dock) and make sure the rw pump is working properly. That would eliminate the pump. Seems like some kind of leak at the thru hull - loose fitting itself, bad strainer connection to the seacock, etc.

Good luck, happy forensics.

Most pumps have reversion, that is slight reverse flow. I'm guessing there is an air leak right at the seacock / thru hull.
Your bucket idea would prove or disprove this theory

[sailorchic34]

Check your crankcase breather for a clog. My thought is the crank case pressure is leaking past the seal on the pump shaft.

[a64pilot]

Originally Posted by sailorchic34

Check your crankcase breather for a clog. My thought is the crank case pressure is leaking past the seal on the pump shaft.

Interesting, but wouldn't it force oil out of both front and rear mains too? Plus it would have to make it's way all the way out against the flow of water?
Interesting thought though, I admit I wouldn't have come up with it

[sailorchic34]

Originally Posted by a64pilot

Interesting, but wouldn't it force oil out of both front and rear mains too? Plus it would have to make it's way all the way out against the flow of water?
Interesting thought though, I admit I wouldn't have come up with it

The air takes the path of least resistance. So the raw water pump seal failed first.

The raw water inlet side is under very low suction pressure. Only a foot or two. A blown seal in the 3 Oclock side of the pump, would allow the combustion gases to enter the raw water. That side of the pump would have a slight negative pressure too.

I expect that some of the air is going out the exhaust. But due to the low pressure at the suction entrance, some pressurized air can get out the intake too. That's about the only way I know you could get bubbles out the intake.

I have seen reverse flow in large chilled water piping (primary secondary system), with part of the flow going one way and the other part going the other way in the same pipe. Unexpected there too.

Easy test would be to open the oil fill and see if the bubbles stop.

[a64pilot]

Originally Posted by sailorchic34

Easy test would be to open the oil fill and see if the bubbles stop.

Yes, plus there would be considerable air flow out of the fill, plus it's a thirty sec job to disconnect the breather tube and check for air flow out of it.


One favor for the person with the problem, when you figure it out please post the answer>

[prof_mariner]

Some really good ideas have been passed along, and I'm a little embarrassed to say this but I think I found the "source" of the bubbles and, if I'm right, it's not the raw water intake. While I was digging through the bilge tracing the raw water line from the through hull to the exhaust I saw something a bit unusual. There's a small rubber hose that runs from the exhaust mixing elbow to the shaft log for the prop shaft. Why that's there is beyond me. But it sure could be the source of the bubbles that are coming from under the boat. This boat (a French built Beneteau) has a Volvo dripless packing gland and I guess the exhaust exit there must have something to do with that? Incidentally, the shaft log and raw water thru hull are right next to each other, so my initial assumption appears to have been completely off.

[a64pilot]

Well it was fun to theroize

[sailor-dave]

ok, now I do have bubbles coming out my raw water but it happens after I crank my engine when trying to start it, after 7-8 sec of cranking I hear and see bubble come up by the raw water intake I'm thinking my water pump is sucking air somewhere between the thru-hull inlet and water pump, any other ideas?

[transmitterdan]

Originally Posted by prof_mariner

Some really good ideas have been passed along, and I'm a little embarrassed to say this but I think I found the "source" of the bubbles and, if I'm right, it's not the raw water intake. While I was digging through the bilge tracing the raw water line from the through hull to the exhaust I saw something a bit unusual. There's a small rubber hose that runs from the exhaust mixing elbow to the shaft log for the prop shaft. Why that's there is beyond me. But it sure could be the source of the bubbles that are coming from under the boat. This boat (a French built Beneteau) has a Volvo dripless packing gland and I guess the exhaust exit there must have something to do with that? Incidentally, the shaft log and raw water thru hull are right next to each other, so my initial assumption appears to have been completely off.

Some water is needed to keep the dripless shaft seal cool. Otherwise is may overheat and start a big leak or worse, it might seize and rip the shaft log apart. I think you should have someone look at it though. Exhaust gas should not be going that way. You could have a problem with too much back pressure in the exhaust system. Could be clogged elbow or bad installation not per engine specifications.